Absorbent material

ABSTRACT

Disclosed is an absorbent material which is used as ink pads of ink-jet and bubble-jet printers, various building materials and materials for civil engineering work, said absorbent material comprising a non-woven fabric containing water-absorptive fibers, wherein a water absorption percentage is not less than 400% by weight and a change in volume is less than 10% in a state where water is absorbed in the amount of not less than 400% by weight.

TECHNICAL FIELD

[0001] The present invention relates to an absorbent material and, moreparticularly, to an absorbent material which is used as ink pads ofink-jet and bubble-jet printers, building materials such as wall andceiling materials, materials for civil engineering work, medicalmaterials such as dental materials, and other materials required to haveexcellent water absorption properties, water retention properties, formstability and easy processing.

BACKGROUND ART

[0002] Absorbent materials having various characteristics have hithertobeen developed according to various purposes. To obtain absorbentmaterials having high water absorption percentage, a method of reducinga density of the absorbent material has been employed.

[0003] However, there is a problem that the volume decreases on waterabsorption when the density of the absorbent material is reduced,thereby causing a problem that water retention percentage decreases byreducing the density and water retention percentage decreases byvibration and so on.

[0004] Therefore, a water-absorptive polymer is mixed with the absorbentmaterial to increase both the water absorption percentage and waterretention percentage. However, there arises a problem that, when awater-absorptive polymer is mixed with the absorbent material, thewater-absorptive polymer falls out on cutting or water absorption. Amethod of preventing this water-absorptive polymer from falling outincludes, for example, a method of preparing a water-absorptive polymerin the form of large particles. However, the water-absorptive polymer inthe form of large particles has a drawback that a change in volume ofthe absorbent material due to water absorption increases when thewater-absorptive polymer in the form of large particles is mixed withthe absorbent material because of large expansion on water absorption.

[0005] As a method of preventing the water-absorptive polymer fromfalling out when a water-absorptive polymer in the form of smallparticles is mixed with the absorbent material, a method of putting amesh-shaped cover over the absorbent material has also been suggested.For example, Japanese Patent Laid-open Publication No. 91052/1981discloses a method of sandwiching a layer containing a water-absorptivepolymer between dry non-woven fabrics. However, this method has aproblem that the position of the absorbent material is not fixed becausethe portion for seaming upper and lower covers is obstructive in thecase of setting in a container box. In this method, there was also aproblem that it is difficult to put a cover over an absorbent materialhaving a complicated cut shape at low price.

[0006] Accordingly, an object of the present invention is to solve theseproblems of the absorbent material of the prior art and to provide anabsorbent material which shows high water absorption percentage and highwater retention percentage as well as small change in volume on waterabsorption and can be easily cut into a complicated form, and which canbe obtained at low cost.

DISCLOSURE OF THE INVENTION

[0007] The present invention provides an absorbent material comprising anon-woven fabric containing water-absorptive fibers, wherein a waterabsorption percentage is not less than 400% by weight and a change involume is less than 10% in a state where water is absorbed in an amountof not less than 400% by weight.

[0008] In this absorbent material, when highly water-absorptive fibersare contained in an amount of 2 to 20% by weight, more excellent waterabsorption properties can be obtained. Furthermore, it becomes possibleto remarkably improve the water retention properties by using highlywater-absorptive fibers of crosslinked sodium polyacrylate.

BEST MODE FOR CARRYING OUT THE INVENTION

[0009] The absorbent material of the present invention is an absorbentmaterial comprising a non-woven fabric containing water-absorptivefibers, wherein a water absorption percentage is not less than 400% byweight and a change in volume is less than 10% in a state where water isabsorbed in an amount of not less than 400% by weight.

[0010] As the water-absorptive fibers, highly water-absorptive fibersare preferably used. The term “highly water-absorptive fibers” usedherein refer to fibers capable of absorbing deionized water in an amounttwenty or more times as much as its own weight. By using the highlywater-absorptive fibers, sufficient water absorption percentage andwater retention percentage can be obtained.

[0011] As the highly water-absorptive fibers, a monomer having thecomposition resemblance to the water-absorptive polymer can be used. Ascarboxylic acid, for example, (meth)acrylic acid, maleic acid, maleicanhydride, fumaric acid, itaconic acid, sorbic acid, cinnamic acid,crotonic acid, -acryloxypropionic acid, and alkali metal salts thereofcan be used. Among them, (meth)acrylic acid is economically preferred.As sulfonic acid, for example,2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)allylsulfonicacid, ethyl sulfonic acid (meth)acrylate, styrenesulfonic acid,vinylsulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and alkalimetal salts thereof can be partially used. The water absorptionpercentage can be improved by containing these sulfonic acids.

[0012] When the highly water-absorptive fibers are crosslinked, it ispossible to use 2-hydroxypropyl (meth)acrylate, hydroxyethyl(meth)acrylate, and their adducts with ethylene oxide and propyleneoxide, and so on as a crosslinking agent. When the amount of thesecrosslinking agent is too large, the water absorption percentage islowered. Therefore, it is necessary to appropriately set the amounttaking the amount of the highly water-absorptive fibers and so on intoconsideration. Furthermore, when a water-absorptive polymer is prepared,a polyfunctional group is used. When the fibers are made, gelationhardly occurs by using those other than the polyfunctional group and,therefore, it is preferred in view of spinning. Crosslinking using theabove crosslinking agent is preferably performed at the time when thefibers were completely formed after the completion of the spinning.

[0013] The highly water-absorptive fibers which are used particularlypreferably in the absorbent material of the present invention include,for example, highly water-absorptive fibers of crosslinked sodiumpolyacrylate, highly water-absorptive fibers obtained by hydrolyzing thesurface layer portion of acrylic fibers, highly water-absorptive fiberscontaining a maleic acid metal salt as a principal component, and so on.

[0014] Since these highly water-absorptive fibers have characteristicssuch as high swelling properties, the amount is preferably not more than20% by weight taking deformation on water absorption into consideration.To obtain sufficient water absorption percentage and water retentionpercentage, the highly water-absorptive fibers are preferably mixed inthe amount of not less than 2% by weight. That is, the amount of thehighly water-absorptive fibers is within a range from 2 to 20% byweight, and preferably from 3 to 10% by weight, based on the totalweight.

[0015] The highly water-absorptive fibers of a crosslinked sodiumpolyacrylate and the highly water-absorptive fibers containing a maleicacid metal salt as a principal component have flame retardance and goodwater retention properties, but have high swelling properties because ofgood water retention properties. Accordingly, there is a problem that,when these highly water-absorptive fibers are contained in a largeamount, the degree of deformation on water absorption becomes large.Furthermore, since the fibers have high water absorption properties, thefibers themselves exhibit spot water absorption properties. Therefore,when the fibers are mixed in a very large amount, the water absorptionrate of the absorbent material itself is decreased by blocking of water.Accordingly, in the case of mixing these highly water-absorptive fibers,the amount is preferably adjusted within a range from 2 to 15% byweight, and particularly preferably from 3 to 10% by weight.

[0016] Furthermore, the highly water-absorptive fibers of thecrosslinked sodium polyacrylate are capable of performing high waterabsorption repeatedly because of very small reduction in waterabsorption percentage due to repeated water absorption, and which issuperior in durability, light resistance and heat resistance on waterabsorption and cause small change with time. Therefore, the highlywater-absorptive fibers are suited as highly water-absorptive fibersused in the absorbent material of the present invention.

[0017] Since the highly water-absorptive fibers obtained by hydrolyzingthe surface layer portion of the acrylic fibers have lower swellingproperties than those of the highly water-absorptive fibers of thecrosslinked sodium polyacrylate and the highly water-absorptive fiberscontaining the maleic acid metal salt as the principal component, thehighly water-absorptive fibers are preferably mixed in the amount withina range from 3 to 20% by weight, and more preferably from 5 to 15% byweight.

[0018] When the absorbent material of the present invention isparticularly applied to electrical parts, it is preferred to mixflame-retardant fibers with them. Examples of the flame-retardant fibersinclude modacrylic, flame-retardant polyester, nylon, wool,flame-retardant rayon fibers, and so on, and these flame-retardantfibers preferably have Loi of not less than 26. Among them, particularlypreferred are modacrylic fibers which easily impart hydrophilicity andare cheap. Such the flame-retardant modacrylic fibers include, forexample, LUFNEN Manufactured by Kanebo Ltd., KANEKARON manufactured byKaneka Corp ,and so on. That is, when using these flame-retardantfibers, there can be obtained those which pass UL Standards (UL'94, MM,Vertical Burning Test, ASTM D3801) in the case of applying to electricalparts.

[0019] The absorbent material of the present invention can be mixed withflammable fibers, in addition to the flame-retardant fibers. When usingthe flammable fibers, it is preferred to mix taking the flame retardantproperties of the flame-retardant fibers and kind of the flammablefibers into consideration. For example, when the modacrylic fibers areused as the flame-retardant fibers, there can be obtained those, whichpass UL Standards, by mixing the modacrylic fibers in the amount of notless than 70% by weight. When an oily agent is coated on the fibers toimpart the hydrophilicity, it is preferred to confirm by actually makinga trial considering that it is influences by the amount of the oilyagent, weight and density of the absorbent material as a product, flameretardance of the other fibers to be mixed, and so on. When using highlyflame-retardant modacrylic fibers, there can be obtained those, whichpass UL Standards, by mixing in the amount of not less than 50% byweight.

[0020] It is also possible to use a pulp as a kind of thewater-absorptive fibers in the absorbent material of the presentinvention. In this case, since the fiber length of the pulp is short,binder fibers are preferably mixed so that the pulp does not fall out.As the binder fibers, there can be used heat-fusing binder fibers whichare core-sheath or side-by-side type conjugate fibers and so on. Thebinder fibers may be made of a homopolymer, but those made of acopolymer are suitable because of large adhesive strength. As the binderfibers, for example, commercially available binder fibers such as EStype fibers manufactured by Chisso Corp., BELLCOMBI manufactured byKanebo Ltd., and the like can be used. In this case, the binder fibersare usually mixed in the amount of not less than 10% by weight,preferably not less than 15% by weight, and more preferably not lessthan 20% by weight. That is, the larger the amount of the binder fibers,the larger the adhesive strength, thereby making it possible to preventthe pulp from falling out. However, since the binder fibers themselvesare flammable, it is necessary to appropriately limit the amount to bemixed considering the amount of the flammable pulp in the case ofimparting the flame retardance to the absorbent material. When using alarge amount of conjugate binder fibers, the conjugate binder fibersform a network and, therefore, it is preferred to retain the shape ofthe absorbent material. Particularly, when the proportion of theconjugate binder fibers becomes 20% by weight, it becomes easier toexert the effect of retaining the shape.

[0021] The fineness and fiber length of these fibers are largelyinfluenced by a method of and an equipment for producing the non-wovenfabric constituting the absorbent material of the present invention. Asthe non-woven fabric constituting the absorbent material of the presentinvention, it is possible to appropriately use those selected fromair-laid, cross-laid, wet, felt, and the like type non-woven fabrics.For example, when the non-woven fabric is a felt, the fineness ispreferably adjusted to not less than 3 denier in average and the fiberlength is preferably adjusted to not less than 50 mm in average. Whennon-woven fabric is an air-laid type non-woven fabric, the fineness ofthe binder fibers is preferably not less than 3 denier so as to increasethe number of constituent fibers, whereas, the fiber length ispreferably from 3 to 20 mm, and more preferably from 5 to 10 mm.Furthermore, in the case of the non- woven fabric produced byconventional carding, cross-laid and needle punching, the fineness andfiber length can be appropriately set according to carding. In thiscase, since the number of needle-punching is smaller than that of thefelt, the shape of the absorbent material is preferably retained usingthe binder fibers by mixing the heat- fusing fibers and the like similarto the case of the non-woven fabric according to the air-laid method.The amount of the binder fiber to be blended varies depending on thedegree of needle punching, but can be made slightly smaller than that inthe case using the air-laid method. The non-woven fabric constitutingthe absorbent material of the present invention may be produced by anymethod described above, but is preferably a felt having large degree ofinterlocking of the fibers without using the binder fibers so as toimpart the flame retardance.

[0022] By using dyed fibers partially as the fibers used in thesenon-woven fabrics, it becomes possible to classify products by coloridentification. In this case, it is preferred to use modacrylic fiberswhich can be easily dyed. In the case of the non-woven fabric producedby the air-laid method, not only classification of products but alsofurther detailed information can be applied by using those colored byprinting as the non-woven fabric and paper to be provided on the upperand lower surfaces.

[0023] The thickness of the absorbent material of the present inventionis preferably adjusted to not less than 1 mm. That is, when thethickness is less than 1 mm, the liquid retention percentage per unitarea is liable to be insufficient.

[0024] The density of the absorbent material of the present invention ispreferably within a range from 0.03 to 0.28 g/cm³. That is, when thedensity is less than 0.03 g/cm³, there is a fear that the degree ofdeformation upon water absorption increases and the liquid retentionpercentage decreases by 10% or more upon vibration. To the contrary,when the density exceeds 0.28 g/cm³, the water absorption percentage isliable to be insufficient. Accordingly, the density of the absorbentmaterial of the present invention is preferably adjusted within a rangefrom 0.03 to 0.28 g/cm³, and more preferably from 0.08 to 0.20 g/cm³.

[0025] The water absorption rate of the absorbent material of thepresent invention is preferably not more than 5 seconds. This waterabsorption rate can be improved by coating an appropriate amount of theoily agent on the fibers. As the oily agent, a general oily agent can beused. Since a cationic oily agent is often inferior in hydrophilicity,nonionic and anionic oily agents are preferably used. The oily agent canbe coated on the fibers on production of the fibers, production of thenon-woven fabric, dyeing, and so on, and the amount varies depending onthe method of producing the non-woven fabric but is preferably adjustedwithin a range from 0.3 to 3.0% by weight. That is, when the amount istoo small, the hydrophilicity is insufficient. To the contrary, when theamount is too large, the operatablity is lowered particularly in thecarding on production of the non-woven fabric.

[0026] In the present invention, the water absorption percentage of theabsorbent material is measured in the following manner. That is, a testmaterial of the absorbent material is allowed to stand on a 40 mesh wirecloth after sufficiently dipped in a liquid such as deionized water.After the test material became free from liquid drop, the weight ismeasured and a percentage (%) is determined by dividing a differencebetween the weight and an absolute dry weight of the same test materialbefore water absorption by the absolute dry weight, followed bymultiplication by 100. When the test material is made of fibers, themeasurement is performed by putting the test material in a tea bag. Whenusing the tea bag, a blank test in a state of putting no test materialis performed and a correction of the water absorption percentage isperformed by subtracting the water absorption percentage of the tea bagon calculation.

[0027] The water retention percentage of the absorbent material of thepresent invention is determined in the following manner. That is, thewater-absorbed test material is dropped, together with the metal cloth,from the height of 30 cm in a state where the test material is allowedto stand on the metal cloth. Then, the water absorption percentage ismeasured again and the water retention percentage is calculated from thewater absorption percentages before and after dropping.

[0028] In the present invention, the water absorption rate of theabsorbent material is measured in the following manner. That is, a testmaterial cut into pieces having a width of 3 cm and a length of 10 cm isdipped in a liquid at the lower portion of 1-2 cm while supportingperpendicularly the test material, and then a time required for theliquid level to reach the height of 5 cm is taken as a water absorptionrate. As a standard solution, deionized water is used.

EXAMPLE 1

[0029] 5% by weight of highly water-absorptive fibers (OASISmanufactured by Technical Absorbent Co.) having a fineness of 9 denierand a fiber length of 51 mm and 85% by weight of flame-retardantmodacrylic fibers (LUFNEN BR manufactured by Kanebo Ltd.) having afineness of 3 denier and a fiber length of 51 mm were mixed, and thencarding, cross-laid and needle punching were performed in a conventionalproduction process of felt to obtain a felt having a thickness of 15 mm,a weight of 2500 g/m² and a density of 0.17 g/cm³ as an absorbentmaterial of the present invention. In the preliminary stage of theproduction of the felt, 2% by weight of an anionic hydrophilic oilyagent was added to the above flame-retardant modacrylic fibers and fiberblending was performed, thereby to improve the water absorption rate ofthe resulting absorbent material.

[0030] The resulting felt was cut into pieces having a width of 3 cm anda length of 10 cm to produce a test material. Using this test material,the water absorption percentage and water absorption rate were measuredby the method described above. As a result, the water absorptionpercentage was 490% by weight and the water absorption rate was 3.9seconds. Furthermore, the thickness of the test material was 16 mm in astate where water was absorbed in the amount of 490% by weight. Afterwater absorption, only the thickness changed without changing the area,and the volume changed by +6.7%.

[0031] Then, the water retention percentage of the test material wasmeasured by the method described above in a state where the testmaterial absorbed water in the amount of 490% by weight. As a result, itwas confirmed that the water retention percentage after dropping is 470%by weight and the water retention is 96%, that is, the water retentionproperties are excellent. Even after this dropping test, no fibers wereremoved from the cut surface of the felt.

[0032] Furthermore, this felt was put in a desiccator containing waterat the bottom and allowed to stand in a temperature controlled bath at30 at 100% RH. Then, the weight after 72 hours was measured. Themoisture absorption percentage was calculated from a difference betweenthe measured weight and absolute dry weight. As a result, high moistureabsorption percentage such as 25% was shown.

EXAMPLE 2

[0033] Absorbent materials No. 1 to 6 were produced in the same manneras in Example 1 except that a mixing ratio of the highlywater-absorptive fibers (A) to the flame-retardant modacrylic fibers (B)used in Example 1 is set as shown in Table 1.

[0034] With respect to the respective absorbent materials, the waterabsorption percentage, the change in volume (change in thickness, areadoes not change) before and after water absorption, the water retentionpercentage, and the water absorption rate were measured. The results areshown in Table 1.

[0035] All of the above absorbent materials No. 1 to 6 passed VO inaccordance with UL Standards. TABLE 1 No. A (%) B (%) Water Change inWater retention Water 1  2 98  410  9 91 6.1 2  4 96  480  4 95 3.3 3  991  620  7 98 4.2 4 14 86  850  9 96 6.5 5  1 99  230  3 73 7.1 6 21 791030 16 90 10.5

[0036] As is apparent from these results, the absorbent materials No. 1to 4 show high water absorption percentage and high water retentionpercentage, small change in volume (change in thickness) before andafter water absorption, and small water absorption rate(second).

[0037] To the contrary, the absorbent material No. 5 was insufficient inwater absorption percentage because the amount of the highlywater-absorptive fibers used is small such as 1% by weight. Furthermore,since the absorbent material No. 6 contains a large amount of the highlywater-absorptive fibers, the water absorption percentage was sufficient.However, the change in volume (change in thickness) was large and thewater absorption rate(second) was large.

[0038] As described above, when the absorbent material of the presentinvention is composed of a felt comprising the highly water-absorptivefibers and flame-retardant modacrylic fibers of the above Examples, thecontent of the highly water-absorptive fibers is preferably adjustedwithin a range from 2 to 20% by weight, and more preferably from 3 to10% by weight, based on the total weight.

[0039] The felt thus obtained described above was cut into variouscomplicated shapes. As a result, it could be easily cut into any shapewithout causing a problem. That is, it was confirmed that this felt hasexcellent cutting processability.

[0040] Furthermore, a change in water absorption change on repeatedwater absorption was examined by using the above absorbent material No.3. As a result, the water absorption percentage was 600% by weight inthe fifth water absorption, 540% by weight in the tenth waterabsorption, and 530% by weight in the twentieth water absorption,respectively, and the water absorption was reduced to only 500% byweight even in the fiftieth water absorption. That is, it was confirmedthat the water absorption percentage is not largely reduced even ifwater absorption is repeated.

EXAMPLE 3

[0041] An absorbent material was produced in the same manner as inExample 1 except that highly water-absorptive fibers (fineness: 5denier, fiber length: 51 mm) obtained by hydrolyzing the surface layerportion of acrylic fibers were used in place of the highlywater-absorptive fibers used in Example 1 and that polyester fibers(Regular/full dull, fineness: 3 denier, fiber length: 76 mm) were usedin place of the flame-retardant modacrylic fibers. As this absorbentmaterial, two kinds of an absorbent material wherein 20% by weight ofthe above highly water-absorptive fibers and 80% by weight of the abovepolyester fibers are mixed (No. 7) and an absorbent material wherein 30%by weight of the above highly water-absorptive fibers and 70% by weightof the above polyester fibers are mixed (No. 8) were produced.

[0042] With respect to two kinds of these absorbent materials, the waterabsorption percentage, the change in volume (change in thickness) beforeand after water absorption, the water retention percentage, and thewater absorption rate were measured in the same manner as in the aboveExamples. The results are shown in Table 2. TABLE 2 No. Water Change inWater retention Water 7 720 9 68 6.1 8 880 4 75 4.8

[0043] It was confirmed that the absorbent materials of this Example areinferior in water retention percentage to those of Examples 1 and 2, butshow high water absorption percentage and small change in volume (changein thickness).

[0044] Industrial Applicability

[0045] As described above, the absorbent material of the presentinvention shows high water absorption percentage and high waterretention percentage as well as small change in volume on waterabsorption, and is superior in cutting processability. Therefore, theabsorbent material of the present invention can be widely as anabsorbent material required to have excellent water absorptionproperties, water retention properties, form stability and easilyprocessing in various fields.

1. An absorbent material comprising a non-woven fabric containingwater-absorptive fibers, wherein a water absorption percentage is notless than 400% by weight and a change in volume is less than 10% in astate where water is absorbed in an amount of not less than 400% byweight.
 2. The absorbent material according to claim 1, which contains 2to 20% by weight of highly water-absorptive fibers.
 3. The absorbentmaterial according to claim 2, wherein the highly water-absorptivefibers are highly water-absorptive fibers of crosslinked sodiumpolyacrylate.
 4. The absorbent material according to any one of claims 1to 3, wherein a density is within a range from 0.03 to 0.28 g/cm³. 5.The absorbent material according to any one of claims 1 to 4, wherein anoily agent is coated on the fibers constituting the absorbent material.6. The absorbent material according to any one of claims 1 to 5, whereina water absorption rate is not more than 5 seconds.
 7. The absorbentmaterial according to any one of claims 1 to 6, which contains not lessthan 50% by weight of flame-retardant fibers.